This thesis examines the fatigue behaviour of FPSO structures. It has been compiled as a result of theoretical, analytical and experimental study. The Finite Element approach has been utilized to analyse the FPSO's structure. It is intended that this particular work will enable further computer simulations for fatigue assessment to be carried out. The thesis starts with the development of the general arrangement, structure and typical details of the City FPSO. The applied loads are then reviewed and this includes the so called static loads due to cargo loading and still water pressure, and the green loads due to dynamic loads induced by the vessel behaviour on waves at sea. Response to local loads such as, external sea pressure, internal pressure due to the cargo and ballast, wave slamming loads, etc. is then determined. The effect of the top structural loads on the FPSO is discussed with some practical calculation of typical topside processing palates loads. SCF evaluation methods are considered together with a discussion of the effect of structural dimensioning of local details, the use of specially performed test results conducted on ship structure. In particular, the structural stress concentration factor at the web-toe associated with the max loading conditions is developed. Confirmation of validity of the SCFs theory is provided from an extensive appraisal of the literature and from laboratory tests of the structure in question. The experimental technique developed in this thesis is based upon geometrical analogy to the simplified Peterson's Neuber notch theory, applied to the system parametric equations of SCFs and the geometric relations. The experimental results are in general accordance with published results. This research includes a calibration method for S-N Curves required for typical fatigue sensitive details in FPSOs. It also provides improved information on the important link between S-N data and finite element analysis for fatigue life assessment using a linear cumulative damage formulation.